1. Field of the Disclosure
The present invention generally relates to processes for joining and microstructure modification of solid materials and apparatus for the same, and more particularly to a new approach to perform such tasks in a controlled manner in the solid state.
2. Description of Related Art
The process of solid state joining of metals involves transfer of atoms from one side of a metal interface to the other. Solid state joining of metals has many attractive advantages over liquid metal joining processes. It is energy efficient, cleaner, free from unsafe liquid metal spurts, does not require filler metal, eliminates hazardous gas emissions, and, therefore, is environmentally more friendly. Solid state joining produces a bonded material without cast metal zone, generally with higher mechanical integrity.
In a diffusion bonding process, the entire workpiece is heated to the bonding temperature and bonding is made over the entire interface at one time. During diffusion/deformation bonding processes, transfer of atoms at the interface occurs by microcreep of the surface asperities concomitant with transport of atoms by diffusion, aided by dislocation motion, grain boundary migration, and grain boundary sliding. In other solid state processes, e.g., ultrasonic welding and friction welding, joining occurs over a small region at one time as the welding zone moves along the interface to create a long welded length for a butt or lap weld.
The friction stir welding (FSW) process has emerged as a viable welding process for aluminum alloys, after the predecessor “friction welding” process (a rubbing process) was found to be restricted. It was reasonable to stir the joining interface to assist rapid transfer of matter from one side of the interface to the other. This process, developed by The Welding Institute and shown in FIG. 1, involves joining of two workpieces held under compressive constraint, by having a rotating tool plunged into the alloy, moving along the interface, to transfer matter from one side of the interface to the other as it moves forward.
The basic understanding of the mechanics of the FSW process is expressed by the following statement from The Welding Institute: “FSW process joins materials by plasticizing and then consolidating the material around the joint line. First, a hole is pierced at the start of the joint with a rotating steel pin. The pin continues rotating and moves forward in the direction of welding. As the pin proceeds, the friction heats the surrounding material and rapidly produces a plasticized zone around the pin. Pressure provided by the pin forces plasticized material to the rear of the pin where it consolidates and cools to form a bond. No melting occurs in this process, and the weld is left in a fine-grained, hot-worked condition with no entrapped oxides or gas porosity.” While this understanding is generally accepted, as shown by the heated region and the mass transfer lines in FIG. 1, the presence of the plasticizing region obscures to some extent the critical features regarding the mechanics of transfer of atoms from one interface to the other, and porosity and cracks can form.
In FSW, metal is physically moved from one side of the interface to the other through shear displacements induced in the workpiece by the rotating tool. The rise in temperature due to frictional heating softens the workpiece to enhance shearing rate. Work hardening, diffusion, and recrystallization across the interface are other concurrent effects. This rise in temperature, an integral part of the process, also accelerates chemical reactions between tool material and the workpiece, adhesion, wear, and damage to the apparatus.
Heat evolution in the welding zone is due to two primary components: (i) friction and (ii) plastic deformation of the metal. These are interdependent variables. A reduction in the flow stress of the workpiece due to heating reduces the frictional shear resistance experienced by the apparatus. Thus, a key to reducing damage to the apparatus due to friction is to develop efficient deformation heating process of the material transferred across the interface. If the efficiency of plastic work input is increased, then available tool power transfers material across the interface with great speed, or in the alternative, the process could be completed at lower power.
Incremental advances in improving the material of construction of the conventional FSW apparatus and in improving complex procedures continue to emerge, but it is becoming increasingly evident that the current processes and apparatus for FSW are less than optimum in various ways. Recent analytical studies show that the energy imparted from the prior art working apparatus is not used efficiently in softening the region of the workpiece, or in transferring matter across the interface, where joining is taking place, but is lost to a great extent in the surrounding material.
For a single pin FSW process, the cross-sectional area of a workpiece increases radially outward from the apparatus axis, the shearing displacement imparted at the interface is rapidly attenuated as a function of radial distance from the interface. This means that a viscous plasticized zone is developed only near the tool interface, even though a plastic zone surrounding the pin extends to a large distance. Thus, the plasticized zone is much smaller in size than the plastic zone, which extends all the way to the elastic-plastic boundary. This situation is illustrated in FIGS. 2a and 2b, which graphically depict the shear strain relative to the distance from the pin. As can be seen, the plasticized region is near the pin, and a great amount of shear strain is developed. However, in the tail of the shear strain distribution (the edge of the elastic plastic boundary), which is a large volume of material, a substantial amount of deformation work is spent away from the tool and away from the interfaces to be joined. Furthermore, because heat dissipation by conduction increases with increasing sectional area from the tool axis in a similar manner, loss of heat around the rotating tool occurs rapidly.
Model calculations are still evolving, but are not sufficiently focused on the problem. Nevertheless, the weakness of the present FSW approach is obvious from the above discussion, i.e., the bulk of the workpiece material uses much of the energy imparted by the apparatus with only small fraction aiding the region to be welded.
Furthermore, FSW of high temperature alloys, such as Fe and Ni base alloys, is considerably more difficult than aluminum because, as the pin tries to “plasticize” a significant volume of the high melting alloy, excessive tool wear and reaction damage occurs at the very high temperatures generated during the process. This leads to prematurely discarding expensive tools. Even though exotic tool materials such as polycrystalline cubic boron nitride, W—Re alloys, and ceramic tooling are being examined by welding high temperature alloys, these expensive tool materials are easily damaged because of their brittleness and the high forces encountered in the process.
Finally, in conventional FSW, pores tend to be created in the weld by the vortices created during stirring. Despite high pressure experienced by the FSW tool, there is not a direct attempt to close porosity.
Stirring in the interior of materials automatically introduces deformation to subdivide grain structure, and provide opportunity to add particulates of a second phase between the grains undergoing shearing and mixing, to produce chemical changes or to form a composite type of microstructure. These aspects are not adequately controlled in prior art processes but can be better controlled by practicing the invention described below.